A liquid crystal display device is widely utilized for a display of a personal computer, television and so forth. The device utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal compound. As an operating mode of the liquid crystal display device, such modes are known as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode.
For such a liquid crystal display device, a liquid crystal composition having suitable physical properties is used. In order to further improve characteristics of the liquid crystal display device, a liquid crystal compound contained in the composition preferably has physical properties described in items (1) to (8) below.                (1) High stability to heat, light and so forth,        (2) a high clearing point,        (3) a low minimum temperature of a liquid crystal phase,        (4) a small viscosity (η),        (5) a suitable optical anisotropy (Δn),        (6) a large dielectric anisotropy (Δ∈),        (7) a suitable elastic constant (K), and        (8) an excellent compatibility with other liquid crystal compounds.        
An effect of physical properties of the liquid crystal compound on the characteristics of the device is as described below. A compound having a high stability to heat, light and so forth as described in (1) increases a voltage holding ratio of the device. Thus, a service life of the device becomes longer. A compound having a high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having a low minimum temperature of the liquid crystal phase such as a nematic phase and a smectic phase, as described in (3), in particular, a compound having a low minimum temperature of the nematic phase, also extends a temperature range in which the device can be used. A compound having a small viscosity as described in (4) decreases a response time of the device.
A compound having a suitable optical anisotropy as described in (5) improves a contrast of the device. In accordance with a design of the device, a compound having a large optical anisotropy or a small optical anisotropy, more specifically, a compound having a suitable optical anisotropy, is required. When decreasing the response time by decreasing a cell gap of the device, a compound having a large optical anisotropy is suitable. A compound having a large negative dielectric anisotropy as described in (6) decreases a threshold voltage of the device. Thus, an electric power consumption of the device is decreased.
With regard to (7), a compound having a large elastic constant decreases a response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Therefore, a suitable elastic constant is required according to characteristics that are desirably improved. A compound having an excellent compatibility with other liquid crystal compounds as described in (8) is preferred because physical properties of a composition are adjusted by mixing a liquid crystal compound having different physical properties.
A variety of liquid crystal compounds having a large dielectric anisotropy have so far been prepared because excellent physical properties that are not found in conventional compounds are expected, and because a suitable balance between two of physical properties required upon preparing the liquid crystal composition is expected from a new compound. Patent literature No. 1 and so forth describe a compound having a 3,4,5-trifluorophenyl group replaced at a terminal, for example, a compound represented by formula (S-1). However, the compound does not have a sufficiently large dielectric anisotropy and therefore a liquid crystal composition containing the compound seems to be quite difficult to meet a threshold voltage required by a commercially available device.

Patent literature No. 2 describes a compound having a 3,5-difluoro-4-trifluoromethylphenyl group replaced at a terminal thereof, for example, a compound represented by formula (S-2), and a compound having a 3,5-difluoro-4-trifluoromethoxyphenyl group replaced at a terminal thereof, for example, a compound represented by formula (S-3). However, the compound represented by formula (S-2) shows a sufficiently large dielectric anisotropy, but has an insufficient compatibility with other liquid crystal compounds, and the compound represented by formula (S-3) has a sufficiently excellent compatibility with other liquid crystal compounds, but no sufficiently large dielectric anisotropy.

Under such circumstances, development has been desired for a compound having excellent physical properties and a suitable balance with regard to items (1) to (8) described above.